Molten metals, such as aluminum, typically contain both dissolved and suspended impurities. Suspended impurities include, for example, the simple and complex oxides, nitrides, carbides, and carbonates of the various elements that constitute the alloy. Dissolved impurities include both dissolved gases and dissolved solids. For example, nitrogen, oxygen, and hydrogen have a high liquid phase solubility in iron. Oxygen is highly soluble in copper and silver. Hydrogen is appreciably soluble in aluminum. Dissolved solid impurities include, for example, sulfur and phosphorous in iron, and alkali elements, such as sodium or calcium, in aluminum.
Fluxing is a general category of processes used to remove both dissolved and suspended impurities by the combination of physical desorption, chemical reaction mechanisms, and floatation of suspended solids. Gas sparging is a commonly employed fluxing process wherein an inert or inert/reactive gas combination is introduced into the melt as efficiently as possible to mix and react with the melt thereby removing impurities. For example, it is well known to disperse chlorine or a reactive chloride gas into a molten metal to form the chloride salt of the metal impurity. The salt rises to the surface of the melt and is thereafter removed. It is also well known, for example, to use fluorocarbons, such as dichlorodifluoromethane, to treat molten aluminum with a reactive gas to reduce the amounts of gas impurities and oxides, along with impurities such as sodium and calcium. Suspended solids are transported to the melt surface by attachment to rising gas bubbles.
One specific use to which gas sparging is useful is purification of molten aluminum. Gas sparging is optimized by dispersing extremely small gas bubbles throughout the molten aluminum or melt. Hydrogen, for example, is removed from the melt by desorption into the gas bubbles, while other alkali elements react with the sparging gas and are lifted into a dross layer by flotation. Dispersion of the sparging gas into the melt is facilitated by a rotating gas distributor, or phase contactor, which simultaneously produces a high degree of turbulence in the melt. Turbulence assures thorough mixing of the sparging gas with the melt which, in moderately turbulent environments, are removed to the melt surface by peripheral interception and equatorial contact, i.e. the particles agglomerate, attach to the gas bubbles, and float to the surface. Impurities removed from the melt by peripheral interception are withdrawn from the system with the dross while hydrogen desorbed from the molten metal leaves the system with the sparging gas.
The process efficiency of a particular phase contactor is related to its ability to maximize liquid and gas interphase interfacial area and to effectively disperse the gas phase throughout the melt volume. Liquid diffusion transport distance refers to the range of hydrogen ion migration in a stagnant melt over a concentration gradient between two stationary points. This quantity is used to estimate liquid phase transport resistance of hydrogen in a particular solution, from a remote site in the melt to a gas bubble in the absence of fluid convection or bulk flow transport. Effective dispersion of the gas heat minimizes liquid diffusion transport distance of cations by the development of a flow field. Additionally, flotation efficiency for removing suspended impurities is inversely proportional to the square of the bubble diameter. Therefore, producing the greatest number of small, dispersed gas bubbles maximizes the physical desorption, chemical reaction, and floatation efficiency.
It is known in the prior art to provide a phase contactor consisting of an impeller fixed to the end of a rotating shaft. The impeller comprises a hub with solid radial vanes projecting from the hub. As the impeller rotates through the melt, a vortex street, i.e. a series of vortices that trail behind an object, is produced at the trailing surfaces of the vanes to generate shear. Using such an impeller, a stream of sparging gas is introduced into the melt as the impeller rotates deep within the melt. Gas buoyancy and the low pressure region created behind the vanes combine to cause the melt and gas to mix. The sparging gas interacts with the vortex street created by each vane and is ejected as small gas bubbles.
The shear field created by the impeller vanes comprises numerous eddies that interact with the subsurface stream of sparging gas to generate small bubbles of gas. Energy to create new surface area is supplied by these eddies. The rotating impeller also imparts radial fluid flow that disperses the bubbles throughout the melt volume. Continuity in an incompressible medium, such as molten metal, results in the unfortunate consequence of an axial flow component to the flow field. As a result, a surface vortex forms, rotating about and flowing downwardly along the impeller shaft, agitating the surface dross and drawing impurities back into the melt.
The most effective rotating impeller phase contactor will operate at high shear, and also promote radial flow. Ideally the phase contactor should also minimize disturbance to the surface dross to prevent recontamination to the gas-treated melt.
It is therefore an object of the present invention to provide an improved rotating impeller head phase contactor which maximizes liquid and gas interphase interfacial area to effectively disperse the sparging gas throughout the melt volume.
It is a further object of the present invention to provide an improved rotating impeller head phase contactor which imparts power to the melt for the purpose of thoroughly mixing the liquid phase with the gaseous phase.
It is also an object of the present invention to provide an improved apparatus including a rotating impeller head phase contactor which creates sufficient turbulence but which minimizes formation of a vortex at the top of the melt around the impeller shaft which would disrupt the dross layer and draw surface impurities down into and recontaminate the melt.